1. Field of the Invention
The present invention relates to a device for controlling variable speed electric motors. The invention more particularly applies to the filtering of radio frequency perturbations generated by an electric motor controlled by a chopper.
Control devices for variable speed electric motors are used, for example, in domestic appliances, such as vacuum cleaners, washing machines, drills, food processors, etc., with power consumption ranging from 100 to 1500 watts.
2. Discussion of the Related Art
FIG. 1 schematically represents a conventional electric motor. Such a motor 1 has two primary windings (or inductors) L1 and L2 associated with a stator 2 directing a magnetic field towards a rotor 3 provided with a third winding (or inductor) L3. Two supply terminals A and B of the motor 1 respectively correspond to one end of each winding L1, L2. The other end of each winding L1, L2 is connected to a brush, respectively B1 and B2, contacting the third winding L3.
Usually, such motors are controlled through an electronic circuit that periodically switches the power supply of the motor so as to fix both its power supply and its speed.
FIG. 2 is an electric diagram which represents a first exemplary device for controlling a variable speed motor such as represented in FIG. 1.
The control device includes a triac 4 having its gate connected to a circuit 5 for selecting the conduction periods of the triac 4 as a function of the desired speed of the motor 1. The triac 4 is disposed between a first terminal A of the motor 1 and a second terminal E of the a.c. mains. A second terminal B of the motor 1 is directly connected to a second terminal F of the mains.
A drawback of such a control device is that the current that flows in the windings of the motor 1 has high variations corresponding to the conduction periods of the triac 4. These high current variations are detrimental to the lifetime of the motor.
A further drawback is that the switching of triac 4 generates perturbations corresponding to harmonics of the supply frequency (for example, 50 Hz). These perturbations generate acoustic noises at the harmonic frequencies. The tolerable limits of these harmonic perturbations are imposed by various standards, for example standard CEI 1000 (EN 60555), which fix, for each harmonic of the supply frequency provided by the mains, a maximum intensity. The harmonic perturbations generated by the triac 4 exceed the limits fixed by this standard, especially for high or intermediate power values of the motor, thus of the speed. This is especially the case for some domestic appliances, for example for the motor of a washing machine during spin-drying cycles.
To reduce both the harmonic perturbations of the power supply frequency and the current variations in the motor, a control device based on a chopper operating in pulse width modulation (PWM) mode of the rectified voltage of the mains is generally used.
FIG. 3 is an electric diagram representing a second exemplary device for controlling a motor having a variable speed such as represented in FIG. 1, using a chopper operating at a fixed frequency in the PWM mode.
The a.c. voltage, drawn from terminals E and F of the mains, is rectified through, for example, a diode bridge 6. A first terminal A of the motor 1 is directly connected to a first output terminal P of the diode bridge 6. A switch K is disposed between a second terminal B of the motor 1 and a second output terminal N of the diode bridge 6. The switch K is controlled by a circuit 7 providing a pulse train at a high frequency (for example 20 kHz), the width of the pulses depending upon the desired speed of the motor 1. The switch K modulates the sine wave signal rectified by the bridge 6 so as to fix both the power supply and the speed of the motor 1. A free wheel diode D disposed between the terminals A and B of the motor 1 reduces the ripples of the current that flows in the windings of motor 1. Generally, a low value capacitor C (approximately 1 .mu.F) is connected in parallel with the diode bridge 6, near the motor 1. The capacitor C is designed to filter the switching peaks of the switch K which is, for example, formed by a power MOS transistor.
Although such a control circuit significantly decreases the harmonic perturbations of the mains frequency, thus complying with the constraints of standard CEI 1000, even at high power, the operation frequency of the chopper (for example, approximately 20 kHz) generates radio frequency perturbations due to the switching of the switch K. Therefore, these perturbations should be filtered so that they are not conveyed through the mains.
For this purpose, filtering cells are conventionally used, for example three cells 8, 9 and 10, connected in series upstream the rectifier 6. These cells are LC cells respectively comprising capacitors C8, C9 and C10, and coils L8, L9 and L10. The number of cells generally depends upon the number of harmonics of the switching frequency of the switch K that is to be filtered out.
A drawback of such a control device is that the number of filtering cells which are required causes the device to be particularly cumbersome and expensive. Indeed, the capacitors and inductors of the LC cells should be sized so that they filter the 20-kHz switching frequency and its harmonics while withstanding the mains voltage (for example 220 volts) and intensities of several amperes because of the power of the motor. The inductances of the coils range, for example, from approximately 10 to 60 mH and the capacitances of the capacitors range, for example, from approximately 1 to 5 .mu.F.